Surgically implantable prosthetic joint

ABSTRACT

A prosthetic joint for the replacement of the metacarpophalangeal and the interphalangeal joints of the hand, and for the knee. The joint includes a ball secured to the distal bone by an intramedullary stem and cylindrical cavity element to receive the ball secured to the proximal bone, also by an intramedullary stem. The cylindrical cavity element is provided with an entrance means so that the joint may be assembled and the stems secured to the two bones.

United States Patent 1191 Schultz SURGICALLY IMPLANTABLE PROSTHETICJOINT [76] Inventor: Robert Jordan Schultz, 64 Little Brook Rd., Wilton,Conn.

[22] Filed: Mar. 24, 1972 [21] Appl. No.:'237,856

OTHER PUBLICATIONS Factors in the Design of an Artificial Hip Joint byJ.

1451 Sept. 25, 1973 Charnley, Proceedings The lnstitute of MechanicalEngineers, Vol. 181, Part 3], 1966-67, pages109-1 1 1.

Primary Examiner-Richard A. Gaudet Assistant ExaminerRonald L. FrinksAttorneyJames M. l-leilman et a1.

[57 ABSTRACT A prosthetic joint for the replacement of themetacarpophalangeal and the interphalangeal joints of the hand, and forthe knee. The joint includes a ball secured to the distal bone by anintramedullary stem and cylindrical cavity element to receive the ballsecuredto the proximal bone, also by an intramedullary stem. Thecylindrical cavity element is provided with an entrance means so thatthe joint may be assembled and the stems secured to the two bones.

4 Claims, 9 Drawing Figures SURGICALLY IMPLANTABLE PROSTHETIC JOINTBACKGROUND OF THE INVENTION This invention relates'to endoprostheticjoints and has particular reference to joints implanted in the body of ahuman being or-animal which can be assembled without special tools andwhich can be forced together by a moderate amount of pressure. Theprosthetic joints are ball and socket arrangements for movement throughmany degrees of freedom, or they may be restricted toa single, or two,planes of motion.

Prosthetic joints have been used to replace defective natural joints inhumans and animals, such joints generally made similar to a hinge, somehaving screw means for securing a bolt or pin to two end portions. Sucharrangements require special tools for assembly and, after joining,present sharp edges to the enclosing soft tissues. Others are of a softsilastic material which acts as a filler, and at times, working as ahinge. One of the features of this invention is to restore a somewhatcondylar motionto the joint which is the more natural type ofmotionrather than a hinge mechanism.

Another feature of the present invention is the ease with which thejoint elements are attached to each other during surgery. The twoelements are attached to the bones via intramedullary stems, and thentheenlargement, i.e. ball of one element is simply pushed into thecylindrical or other shaped cavity of the other element, relying uponthe basic anatomy of bone and soft tissues plus the ball and socketarrangement to hold the two elements in condylar relationship. After theball enlargement is positioned in the channelcavity, the stem 12 isforced into the medullary canal whereby the overhanging bone structurecloses off the top of the channel (see FIG. 2).

Another feature of the invention is the minimal exposure of sharppoints, edges, and protruding elements which might cause damage to thesurrounding soft tissue. This is accomplished by having an offset stemsuch that the head of the metacarpophalangeal component can be buriedbeneath the ledge of bone.

Another feature isanextension guard (20) to limit extension of thejointfrom exceeding the neutral positiomand to prevent the ball frombeing dislodged from the socket element by tilting the ball downwardlyas in asee-saw action.

Another feature of the invention is that there is an attempt to restorecondylar motion in flexion and extension which more closely resemblesthe normal joint motion.

For a better understanding of the'present invention, together with otherdetails and features thereof, reference is made to the followingdescription taken in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE FIGURES FIG. 1 is a side view of the-hand, withsome parts in section showing several prosthetic joints installed in themetacarpophalangeal and proximal interphalangeal joints.

FIG, 2 is a side view of one ofthejoints shown in FIG. 1.

FIG. 3 is an end view of the cylindrical cavity portion of the jointshown in FIG. 2. In this view the enlarged portion has been removed.

FIG. 4 is a cross sectional view of the socket or channel portion ofFIG. 2. This section is taken along a vertical median plane of thatfigure.

FIG. 5 is a top view of the ball enlargement portion of the joint.

FIG. 5A is an end view of the enlarged portion of the ball componentwith the ball offset from the axis of the stem and the bearing portionof the stem formed with a concave surface.

FIG. 6 is an end view of the ball portion of the joint with the ball inthe midline.

FIG. 7 is a top view of the cylindrical cavity portion of the joint.

FIG. 8 is a side view of a'ball and socket joint applied to'the knee.This is an alternate form with reinforcements added to permit the weightofthe upper portion of the body to be supported by the joint and the at-Y tached bone structure.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the figures,the joint comprises a socket element 10 and a ball element 11. Thesocket element 10 includes an offset intramedullary stem 12 which isinserted into the medullary canal of the bone for permanently securingone end of the joint. The socket element 10 also includes a cylindricalor other shaped channel 14 terminated by a curved or spherical surface13. One end of the channel is open to provide an entrance foran-enlargementor ball proper 15 secured to the ball element 11 by areduced diameter cylindricalneck 16. Channel 14 adjoins slot 17. Theslot 17 is narrower than channel 14 whereby to retain ball 15 in channel14 and thus provide condylar motion asneck 16 moves along slot 17. Theslot 17 can be wider than the diameter of the neck 16 to provide alimited second degree of motion to'the distal portion of the joint. Theball element 11 alsoincludes an intramedullary stem to enter themedullary canal of the distal bone. While neck 16 and ball 15 may becentrally positioned on the end of intramedullary stem 18, it maypreferably be offset slightly toward the top of the latter stem as canbe seen in the modification illustrated in FIG. 5A as mentioned later.

During the assembly of the joint, the ball 15 is entered at the top ofthe channel 14, after which the ball is then free to move up and downwithin the limit of channel 14. A small extension 20 is provided at theupper surface of the socket element to restrict the hyperextension ofthe joint (FIG. 4). The type of joint described above gives the patientfreedom of movement. However, the joint permits a more specializedmovement resembling the motion of a bony condyle. The'ball has freedomof motion in extension and flexion within the cylinder 14 to effectcondylar motion to compensate for the unequal distance in the horizontaland vertical axes of the normal condyle.

Both portions of the joint may be made of stainless steel or plasticsuch as high molecular weight polyethylene but the preferred materialsare an ultra high molecular weight polyethylene for the proximalcomponent i.e., socket element 10, and a nonreactive metal, such asstainless steel, for the distal component i.e., the assembly 11.

Before the stems 12 and 18 are entered into the bone canals, a cavityforming tool with'raised rasp portions having the same dimensions as theintramedullary stems is employed to form a channel in the bone.

The joint shown in FIG. 8 is a slightly modified form for the knee andcomprises a proximal socket portion with an intramedullary stem 22 andan end portion 23 in firm contact with the end of the bone. End portion23 receives the ball 15 of the distal component. This end portion ismade of high molecular weight polyethylene and includes the cylindricalopening 14 and the slot 17 as described above. The distal portion of thejoint is the same as described above except that the stem 28 is formedwith an enlarged ridge 30 which provides a broader contact base for thedistal bone to support the weight of the body when the patient is in astanding position for a firm weight sustaining contact is necessarybetween the joint and the bone structure.

As mentioned above, referring to FIGS. and 6 the cylindrical neck 16with the ball could be offset from the center of the base stem 18instead of being centrally located both vertically and horizontally. Thepreferred offset position would be toward the top portion as seen inFIG. 5A whereby optimum results have been achieved in certain cases.Likewise, the forward or thick end of the base stem 18, at cylindricalneck 16, in its long direction as illustrated by 18A may be curved,being slightly concave in relation to ball 15A instead of beingperfectly flat. Instead of a ball or sphere 15, and a cylindricalchannel or socket 14, other types of enlarged areas and geometricalshapes obviously may be used as long as the two parts are compatible andcoact together. Many other modifications and equivalents are readilyapparent, all of which are included herein.

The embodiment of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. An implantable prosthetic joint of the ball and socket typecomprising? a first element including an in tramedullary stem foraffixing said element into the medullary canal of the bone, a neckportion of reduced diameter at one end of the stem, and a ball securedto the neck portion and being of greater diameter than said neckportion; a socket element including a second intramedullary stem havinga longitudinal axis for affixing said socket element in another boneportion, a ball retaining means on the socket element and including anelongated cylindrical channel extending'transverse to said longitudinalaxis retaining the ball while providing rotational pivoting and linearmotion of the ball, said channel being of uniform diameter along a majorportion of its length and terminating at one side of the retaining meansin a hole having a diameter at least equal to said ball where the ballis entered when first assembled, an elongated slot intersecting saidchannel slidably receiving said neck portion, said slot being narrowerthan said channel and an extension at said one side of the ballretaining means for limiting the pivoting motion of the first element.

2. A prosthetic joint as claimed in claim 1 wherein said neck portion isformed of integral one-piece construction with the ball andintramedullary stem.

3. A prosthetic joint as claimed in claim 1 wherein the socket elementof the joint is made of a plastic material such as polyethylene and theball portion of the joint is made of a nonreactive material such asstainless steel.

4. A prosthetic joint for the hand as claimed in claim 1 wherein theslot is wider than the diameter of the neck to provide a limited seconddegree of motion to the distal portion of the joint.

1. An implantable prosthetic joint of the ball and socket typecomprising: a first element including an intramedullary stem foraffixing said element into the medullary canal of the bone, a neckportion of reduced diameter at one end of the stem, and a ball securedto the neck portion and being of greater diameter than said neckportion; a socket element including a second intramedullary stem havinga longitudinal axis for affixing said socket element in another boneportion, a ball retaining means on the socket element and including anelongated cylindrical channel extending transverse to said longitudinalaxis retaining the ball while providing rotational pivoting and linearmotion of the ball, said channel being of uniform diameter along a majorportion of its length and terminating at one side of the retaining meansin a hole having a diameter at least equal to said ball where the ballis entered when first assembled, an elongated slot intersecting saidchannel slidably receiving said neck portion, said slot being narrowerthan said channel and an extension at said one side of the ballretaining means for limiting the pivoting motion of the first element.2. A prosthetic joint as claimed in claim 1 wherein said neck portion isformed of integral one-piece construction with the ball andintramedullary stem.
 3. A prosthetic joint as claimed in claim 1 whereinthe socket element of the joint is made of a plastic material such aspolyethylene and the ball portion of the joint is made of a nonreactivematerial such as stainless steel.
 4. A prosthetic joint for the hand asclaimed in claim 1 wherein the slot is wider than the diameter of theneck to provide a limited second degree of motion to the distal portionof the joint.